The present invention relates to an apparatus for and a method of making and collecting fine fibers, e.g. microfibers, and, in particular, to an apparatus for and a method of making and collecting fine fibers wherein the high energy, high temperature gas streams carrying the fine fibers are cooled, without wetting the fine fibers, to reduce the temperatures at and surrounding the primary and secondary fine fiber collection equipment used in the process.
Fine fibers, such as microfibers having mean diameters between about 0.5 and 2.0 microns and other fine fibers having mean diameters of about 7 microns or less, are made from glass, high temperature resistant ceramic materials, organic (carbon based) materials, polymers and other fiberizable materials. These fine fibers are typically made by a process, such as a flame attenuation process, wherein filaments or fibers are heated and attenuated by a high energy hot gaseous blast and carried by a gaseous stream of combustion gases and inspirated air to a collection surface where the fine fibers are separated from the gaseous stream and collected on the collection surface. Typically, the fine fibers are collected as a mat of randomly oriented fibers which is later removed from the collection surface and delivered to a windup mandrel or pneumatic conveying system for packaging or further processing.
One flame attenuation process used to form such fine fibers involves the formation of continuous, relatively large diameter filaments which are formed by pulling the filaments from fiberizing orifices in the bottom of a heated pot or melter, containing a molten fiberizable material, by pull rolls. The continuous filaments are then fed into the hot, high energy gaseous blast of a burner where the continuous, relatively large diameter filaments are attenuated, formed into discrete length fibers, and carried through forming tubes toward a primary fine fiber collection equipment by the combustion gases from the burner and inspirated air. While other filtration means can be used to separate the fine fibers from the gaseous stream of combustion gases and inspirated air, typically, the primary fine fiber collection equipment is a rotating metallic collection drum with a perforated, cylindrical collection surface.
A negative pressure is created within the rotating metallic collection drum by means of an exhaust system, connected to the interior of the collection drum, which withdraws gases from the interior of the collection drum. The negative pressure within the collection drum draws the gases of the fine fiber containing gaseous stream through the perforations in the collection surface thereby separating the gases from most of the fine fibers in the gaseous stream which are collected into a mat of randomly oriented fibers on the primary collection surface. The mat of fine fibers is subsequently removed from the primary collection surface by a doffing and smooth roll assembly and delivered to a windup mandrel or a pneumatic conveying system for packaging or further processing.
In the past, the exhaust gases from the primary collection equipment, carrying fine fibers not removed from the gasses by the primary collection equipment, have been passed through wet scrubber exhaust gas abatement equipment to remove fine fibers from the exhaust gases and clean the exhaust gases. The cleaned gases were then discharged to the atmosphere through an exhaust stack. The use of wet scrubbers, as exhaust gas abatement equipment in a fine fiber production process, causes a number of problems. Typically, the volume of air and combustion gases passing through the primary collection equipment and into the exhaust system to be cleaned by the exhaust gas abatement equipment ranges from about 20,000 to about 60,000 ACFM (actual cubic feet per minute). A wet scrubber used as the exhaust gas abatement equipment in such a process must be operated at a high pressure drop across the scrubber, about 16 inches of water column, to achieve a filtration efficiency of about 90% for fiber five microns in diameter and above. Thus, the operation of a wet scrubber in such a process is expensive. In addition, fine fibers such as glass microfibers, are hydrophilic and once these fibers are wetted by the scrubber, the fibers are no longer useable in a product and become scrap. Another problem associated with the use of a wet scrubber to clean exhaust gases in a fine fiber production process is the creation of clumps of wet fibers in the exhaust system which can be discharged through the exhaust stack. The water used in a wet scrubber absorbs volatiles from the exhaust gases and becomes waste water which must be treated and from an aesthetic point of view, the steam discharged from the exhaust stack of the process can be unsightly.
In fine fiber production processes, such as the flame attenuation process discussed above, not only does the process have to handle large volumes of gases, e.g from about 20,000 to about 60,000 ACFM traveling at speeds of up to about 500 feet per minute, but the ambient or surrounding temperature in the fine fiber collection chamber at the primary collection equipment, due to the high temperatures of the combustion gases typically ranges from about 350.degree. F. to about 550.degree. F. and the temperatures of the exhaust gases passing from the primary collection equipment to the exhaust gas abatement equipment ranges from about 300.degree. F. to about 500.degree. F. The temperatures are typically at the higher end of the ranges in the hotter summer months.
Thus, there has been a need to cool the gaseous stream of combustion gases and inspirated air carrying the fine fibers to the primary collection equipment so that the fine fiber collection equipment can be operated at lower temperatures to prolong the service life of seals, the doffing roll and other parts of the primary collection equipment adversely effected by high temperatures. There has also been a need to cool the exhaust gases so that exhaust gas abatement equipment other than wet scrubbers, such as dry filter drums, can be effectively used to remove fine fibers, remaining in the exhaust gases, from the exhaust gases without wetting the fine fibers. By removing the fine fibers from the exhaust gases without wetting the fine fibers, the fine fibers thus removed from the exhaust gases can be collected and used in products. The exhaust gas abatement equipment can thereby function not only as a means to remove fine fibers from the exhaust gases but also as a secondary collection system for the process to reduce the amount of scrap fibers produced by the process.